1. Field of the Invention
The present invention relates to an apparatus for running tubular members such subsea casing strings in a wellbore. More particularly the present invention relates to a wiper plug and internal drop ball mechanism that may be used in conjunction with the running and cementing of such tubular members in a wellbore.
2. Description of Prior Art
In oilfield applications, a xe2x80x9ccasing linerxe2x80x9d and a xe2x80x9csubsea casing stringxe2x80x9d are tubular members which are run on drill pipe. The term xe2x80x9ccasing linerxe2x80x9d is usually used with respect to drilling operations on land, while the term xe2x80x9csubsea casing stringxe2x80x9d is used with respect to offshore drilling operations. For ease of reference in this specification, the term xe2x80x9ccasing linerxe2x80x9d is used to denote either a xe2x80x9ccasing linerxe2x80x9d or xe2x80x9csubsea casing string.xe2x80x9d
Prior art drop ball-actuated float equipment for use in cementing casing liners in place includes, for example, a float shoe or float collar which has one or more flapper valves and which is located at or near the bottom of the casing liner. The flapper valve or valves are conventionally held open by a breakable plastic tab which is actuated (i.e., broken) by a drop ball when the cementing operation is to begin. The industry has traditionally used systems where a drop ball is released at the surface, and the drop ball must be small enough in diameter to pass through the smallest restriction in the drill string, which usually is the diameter of the bore in the running tool. The size of such restrictions has, therefore, limited the maximum size of the opening in a float collar or shoe. In the case of 13xe2x85x9cxe2x80x3 casing liner, the maximum diameter of a drop ball is somewhere between 2 to 3 inches. Due to the small diameter bore of traditional float equipment and the highly contaminated environment in which such equipment is used, the valves in traditional float equipment tend to become plugged with cuttings and contaminants.
As a casing liner is lowered into the wellbore, the fluid in front of the casing liner must be displaced to flow through the opening in the float equipment as well as around the outside annulus defined by the wellbore and the casing liner. The flow resistance of the two flow paths may be high and thus causes a pressure known as surge pressure to build up below the casing liner. This surge pressure can: (a) cause damage to the formation; (b) result in loss of expensive drilling fluid; and (c) result in the casing liner sticking against the side of the borehole, which means the casing liner does not go to the bottom of the hole.
U.S. Pat. No. 5,960,881, which is incorporated herein by reference, discloses a downhole surge pressure reduction system to reduce the pressure buildup while running in a tubular member such as a casing liner. The system is typically located immediately above the top of the casing liner. Nonetheless, any plugging of the float equipment at the lower end of the subsea casing string can, and very well may, render the surge pressure reduction system of the ""881 patent ineffective.
The method and apparatus according to the present invention overcomes the plugging problem and allows enhanced passage of fluid through the tubular member and into the surge pressure reduction tool.
In accordance with the present invention, apparatus is provided for running a tubular member through a wellbore containing drilling fluid using a drill string.
Apparatus in accordance with the present invention comprises a running tool connected to the top of the tubular member having an axial bore therethrough.
Apparatus in accordance with the present invention further comprises a wiper plug assembly which is releasably suspended from a running tool for the wiper plug within the tubular member and having a receptacle sleeve to receive a drill pipe dart. During cementing operations, the wiper plug assembly receives the drill pipe dart and is released from the drill string at the top of the tubular member. The wiper plug assembly is then pumped downward forcing cement out of the bottom of the tubular member and into the annulus between the tubular member and the borehole.
One end of the running tool for the wiper plug is connected to the running tool attached to the tubular member. The running tool for the wiper plug comprises an axially indexing sleeve and a plurality of wedge-shaped fingers which releasably engage the wiper plug receptacle sleeve. During running in of the tubular member, the drilling fluid flows from the casing liner upward through the ports between the fingers and into the void above the wiper plug fins. To isolate the wiper plug fins from internal pressure during cementing operations, the drill pipe sleeve is indexed axially downward to block the ports between the fingers.
Apparatus in accordance with the present invention also comprises a drop ball sub attached to and below the wiper plug assembly within the tubular member. The drop ball sub releases a float equipment actuator ball which is larger in diameter than the smallest restriction in the drill string. When released, the actuator ball drops to the bottom of the tubular member where it actuates float equipment. Once actuated, flapper valves in the float equipment prevent the back flow of cement traveling downward through the tubular member.
Apparatus in accordance with the present invention may further comprise a surge pressure reduction device or diverter tool connected between the drill string and the running tool. When the diverter tool is in an open port position, the drilling fluid may flow upward from inside the diverter tool into the annulus between the casing cemented in place and the drill string. When in a closed port position, the device provides passage for fluid to travel downward through the drill string.